Light scattering smoke detectors are in common use and are based upon the principle that the presence of smoke or other particulate matter in a projected light beam will cause scattering of the light beam. Such smoke detectors have a light emitter broadcasting or projecting a light beam into a smoke chamber. If a suitable detector is placed in an area within the smoke chamber where the direct light from the projected light beam does not fall upon the detector but rather only scattered light from the beam, then the detector can be calibrated to determine the amount of particulate matter present in the smoke chamber based upon the amount of scattered light detected. Once a certain threshold level of light falling on the detector is reached or exceeded, such that the output of the detector exceeds a preset value, the smoke detector alarm circuits are activated.
In light scattering smoke detectors the presence of extraneous particulate material such as dust within the smoke chamber will cause a degree of light scattering and can raise the background level of the smoke detector and give rise to false alarms. Dust accumulation is the predominant degradation mechanism in optical scattering smoke detectors and results in an increase in the background signal while the threshold signal level required to set off the alarm stays constant. This results in a reduction in the difference between these levels and thus increases the sensitivity of the detector.
Other potential problem areas for light scattering smoke detectors relate to component degradation or the presence of materials in the atmosphere which may cause a film to be deposited upon the light elements, the emitter and detector. The degradation of the components or the presence of such films may cause a reduction in the intensity of the light beam from the emitter as well as a reduction in the detected light level of the scattered light beam by the detector. In either or both of these situations, the sensitivity of the detector would be reduced such that higher smoke levels would be required to reach the threshold signal level and trigger the alarm. This could lead to potentially increased risk of loss of life and property damage as the fire condition would be further advanced prior to detection.
There have been a number of designs of light scattering and other types of smoke detectors developed which have utilized various means for testing for signal level required to activate the alarm. For example, U.S. Pat. No. 3,868,184 describes providing a wire of a size to mimic the amount of light scattering produced by 2 to 10% per foot smoke obscuration which can be rotated into the light beam to test for the sensitivity of the smoke detector. Also, U.S. Pat. No. 5,170,150 describes the use of an external device to rotate a reflective element into the light beam to test for sensitivity.
There have also been some attempts in the past to design smoke detectors which measure background and when it has degraded to too large (or too small) a value so as to substantially shift the detector's alarm point, a trouble indication is given.
U.S. Pat. No. 4,930,095 by Yuchi describes an addressable smoke detector which "corrects" for optical sensitivity changes with a reference light source but ignores background changes. An additional light emitter broadcasting in close proximity to the photodetector is set to produce a receiver output signal equal to that attained from the main light emitter and photodetector at the smoke alarm point. When the panel sends a test command to a particular unit, the added light emitter is turned on and the receiver output signal is compared to the original value. Differences are normalized out by rescaling the output transducer sensitivity. The patent ignores the possibility of background change causing the measured response change with the Test light emitter activated.
U.S. Pat. No. 4,595,914 by Siegel describes an ionization detector with a clock to periodically shunt the ion chamber circuit with fixed resistors to impose a minimum and then a maximum sensitivity test with the alarm sounder being inhibited during these self test levels which bracket the intended alarm sensitivity. Response of self test outside the bracketed range results in a unique trouble signal.
U.S. Pat. No. 4,965,556 describes an ionization detector which automatically performs the test for minimum smoke sensitivity equivalent to the manual push button test at the same time each week so as to relieve the resident from having to perform this test. Occupants will come to expect this test and not be bothered by the alarm sound. Failure of the unit to respond to the self test will cause the occupants to repair the unit.
U.S. Pat. No. 4,687,924 by Galvin; U.S. Pat. No. 4,695,734 by Honma; U.S. Pat. No. 4,728,935 by Pantus; U.S. Pat. No. 4,749,871 by Galvin, and; U.S. Pat. No. 4,827,247 by Giffone all describe projected beam detectors with periodic self test where the received signal is compared to the original value at time of installation (or initiation). Compensation is applied in small steps to restore original sensitivity. Projected beams suffer mainly from loss of signal with time due to contamination of optical surfaces although they are configured to compensate for signal increase. The correction time base is long and correction is made in very small steps to prevent masking a slow smouldering fire's long smoke density buildup.
U.S. Pat. No. 4,647,785 by Morita and U.S. Pat. No. 5,247,283 by Kobayashi describe adding extra optical components as a check on the main smoke detecting pair and presume that the extra pair will somehow be immune to the degradation to which the main pair are subjected. Kobayashi also describes transmitting through the insect screen to check for excessive dust buildup.
In some of these devices the degree of smoke alarm point shift may be inferred from the background measurement and may be indicated by annunciation. Correcting these prior art detectors which have shifted in sensitivity generally requires their removal from the installed location and servicing and readjustment possibly at the factory or other service location.